The present invention relates to x-ray computed tomography machines (CT) and specifically to a method of calibrating CT data when acquired at different sampling rates.
X-ray computed tomography is a well known procedure for creating cross-sectional images from computer processed x-ray projections taken along the plane of the cross section. In a typical CT machine, an x-ray tube is mounted on a rotatable gantry to project the fan beam of x-rays at a patient through a xe2x80x9cslicexe2x80x9d from a variety of angles. The x-rays are received after passing through the patient by a multi-element detector to provide a measurement of x-ray attenuation along a variety of rays of the fan beam (xe2x80x9cprojectionsxe2x80x9d). The attenuation signals from the elements of the multi-element detector are sampled and digitized by a data acquisition system.
Digitized projections collected at a range of angles about the patient, typically no less than 180xc2x0 plus half the fan beam angle, are collected in a xe2x80x9ctomographic projection setxe2x80x9d and reconstructed according to well known techniques in the art, such as filtered back projection, into an image of a cross section of the patient along that slice.
The mathematics of computed tomography reconstruction require that each detector be extremely stable so that attenuation signals over time are the same when identical x-ray flux is received by those detectors. To realize this stability, the detector elements are manufactured to have similar electrical characteristics and remaining variations are accommodated by means of one or more xe2x80x9ccorrection vectorsxe2x80x9d.
The correction vectors provide a value for each detector element which may be subtracted from or multiplied by corresponding attenuation values (xe2x80x9cscan valuesxe2x80x9d) acquired by the detectors to correct the attenuation values for detector-to-detector variation. The correction vectors are updated at different intervals. Prior to every scan, an xe2x80x9coffset vectorxe2x80x9d is measured that corrects signal offsets such as from xe2x80x9cdark currentsxe2x80x9d that occur in detectors in the absence of any received x-rays and contains values subtracted from the attenuation values to remove offset. At the time of the scan, a xe2x80x9creference normalize vectorxe2x80x9d is produced based on a signal received at a reference detector. The vector corrects for variations caused by changes in x-ray tube current.
On a daily basis, an xe2x80x9cair calibration vectorxe2x80x9d is measured which corrects signal scaling from a variety of possible sources including changes in x-ray tube voltage, aperture, focal spot size, filtration and sampling rate. The air calibration vector is measured with nothing in the x-ray beam, prior to scanning patients. Far less frequently, xe2x80x9cbeam hardeningxe2x80x9d and xe2x80x9cprimary speedxe2x80x9d correction vectors are measured, the latter which is a function of the detector and does not change for a give detector. These correction vectors are typically measured rarely, once at the time of manufacture and thereafter only at major service intervals, for example, when the x-ray tube or filters are replaced.
Current CT machines allow for selection from a variety of scanning speeds. High scanning speeds may be desired for images where organ or patient movement can be a problem and low signal to noise ratio can be tolerated. Slower scanning speeds are used where motion is less of a problem and high signal to noise ratio images are needed. Each of these scanning speeds may require the use of a different sampling rate of the attenuation signals from the elements of the multi-element detector.
Variations in the sampling rate can significantly affect the air calibration vector. Accordingly, the calibration vector must be measured for each possible sampling rate, significantly increasing the time required to do this daily calibration procedure.
The present inventors have recognized that a simple relationship may be developed between the values of the calibration vectors at different sampling rates. This relationship, which may be determined by executing a series of stationary air-scans at different sampling rates, may be used to modify a limited set of calibration vectors taken at a base sampling rate, for use with any sampling rate.
Generally, the present invention provides a method of calibrating attenuation signals obtained from a multi-element x-ray detector used in an x-ray computed tomography machine where the attenuation signals indicate the strength of x-rays received from an x-ray source after the x-rays pass through a measurement volume. The signals are sampled by a digital acquisition system at different sampling rates. For each of a plurality of different sampling rates including a base rate, the multi-element detector is used to acquire an air-scan vector of signals when the measurement volume is empty of an object to be imaged. The multi-element detector is then used to acquire at a given sampling rate, a tomographic projection set of signals when the measurement volume includes an object to be imaged. A sampling rate correction vector is generated being a function of the air-scan vector for the base rate and the air-scan vector for the given sampling rate and this is used to modify a calibration vector. The modified calibration vector is applied to the tomographic projection set.